The first scan inside a tight ball of fire ants shows that crisscrossing grips and the absence of slackers makes living rescue rafts.

“Imagine if you had 100,000 people trying to build a raft within a few minutes,” says mechanical engineer David L. Hu of Georgia Institute of Technology in Atlanta. An individual ant isn’t an innately buoyant material for a raft. It’s slightly denser than water. But during floods, a writhing ball of about 100,000 Solenopsis invicta fire ants needs only two or three minutes to arrange itself into a raft that can float for months.

The ants also grip themselves into bridges, towers and other structures.

In small balls of 110 ants, CT scans find each ant participating in an average of 14 connections, as gripper or grippee, Hu and his colleagues report in the June 15 Journal of Experimental Biology. “There’s no freeloading,” he says.

The fire ants orient themselves (sort of) perpendicular to each other, with little ants snuggled into gaps left by big ones. Crisscrossing legs trap air bubbles and leave only small openings, like pores, in the mass of bodies. “The smaller the ants make the pores, the harder it is for water to get pushed inside,” Hu says.